Method of making dense refractory objects



Jan. 16, 1962 w. D. ANDERSON ETAL 3,

METHOD OF MAKING DENSE REFRACTORY OBJECTS I Filed Sept. 30, 1958 a Ff;

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BY en/ Arrow/5 United States Patent Q 3,016,598 IVETHOD OF MAKING DENSEREFRACTORY OBJECTS William 1!). Anderson, La Crescenta, Calif, William9.

Brandt, Denver, Cold, and Leon J. Le Clercq, Glendale, and Jarvis .l.Fargo, La Crescenta, Calif., assignors to Gladding, McBean & (30., LosAngeles, (Iaiitl, a corporation of Qalifornia Filed Sept. 30, 1958, Ser.No. 764,352 6 Claims. (Cl. 25-156) The present invention relates to amethod of making very hard, dense, uniformly compacted, strong objectscomposed essentially of very refractory oxides. The resulting bodies areof very high specific gravity or density, are extremely strong, have avery minor and insignificant porosity or absorption and arecharacterized by uniform, excellent electrical transmissioncharacteristics and a high dielectric constant.

Modern technology requires various reactors, reaction chambers and otherarticles which are capable of withstanding extremely high temperatureswithout any appreciable loss in strength. These refractory articles andobjects cannot be made of metals since most metals known to us losetheir strength very rapidly at temperatures of say 2000" F. in manynuclear reactions, it is necessary to conduct the reaction in chambers,retorts, tubes and the like at temperatures on the order of 3000" F. andhigher. Furthermore, with the advent of flight into outer space and thedesirability of reentry into the atmosphere, it is necessary to providecomponent parts for missiles as well as other aircraft which are capableof withstanding tremendous thermal shocks and maintaining strength atvery high temperatures. In many instances these various articles andcomponent parts of guided missiles and other aircraft must have verygood electrical characteristics for the purpose of permittingtransmission of S.H.F., radar and other wave forms of energy which areused in orienting, guiding or controlling the flight of such missilesand aircraft. in order to attain all these desirable characteristics ithas been found necessary to employ refractory materials or oxides andnot metals. The present invention is directed towards methods of makingthese extremely strong, high temperature resistant and electricallysatisfactory elements and components, even though some of the componentsare large in size.

It is recognized that ceramic objects have been made heretofore by anumber of different methods including dry pressing, wet pressing,jiggering, and casting. None of these methods are capable of producingarticles of the required size, density, homogeneity and temperatureresistance required of the articles to which this invention is directed.Most of the prior methods employed in the ceramic industries require theuse of plasticizers, plastic clays and other substances which containalkaline components, such as alkaline components reducing the meltingpoint of the more reactive constituents of a ceramic body and therebybonding such constituents together. In order to attain the dielectricconstants, hardness, density and resistance to thermal shock which isrequired for the objects of the present invention, plastic clays andreactive alkalies should be virtually absent.

Generally stated, therefore, the present invention is directed to amethod of making articles composed essentially of aluminum oxide,titanium oxide, magnesium oxide or zirconium oxide. Although themetallic ions mentioned constitute the major constituents (over 95% andotter 98%99%) of the finished article, and are generally utilized in theform of oxides, they may also be derived, in part at least, from theirhalides, hydroxides, sulfates and other salts. In addition, thecompositions used in making these articles may contain mineralizing iceconstituents which exert their effect during firing on crystal growth,consolidation of crystals without entrapped bubbies or voids, andenhance the density, strength and re fractoriness of the finishedarticle. Magnesium chloride or hydroxide, barium sulfate, lithiumchloride, calcium fluoride, and other halides of these and other metalssuch as manganese and chromium may be used both as mineralizers andsources of essential refractory oxides. The method of manufacturehereinafter disclosed is distinguished by the formation of an articlefrom such typical ceramic refractory oxides in a state of extremely finesubdivision, these refractory oxides being deposited as an atomizedsuspension in a volatile nonaqueous vehicle upon a hard, impervious,rigid core or mandrel to form an article having the desired wallthickness. The oxides thus deposited upon the mandrel are dried andrendered extremely compact and dense by the application of fluidpressure. Thereafter the formed articles are removed from the centralmandrel and suitably tired as will be described in greater detailhereinafter.

It is an object of the invention therefore to disclose and provide amethod of forming refractory, strong and dense objects composedessentially of refractory oxides.

A further object of the invention is to disclose and provide a method ofmaking large accurately dimensioned objects having a density in excessof 92% of the density of the pure refractory oxide employed in itsmanufacture, such formed objects also being characterized by extremelyhigh strength, resistance to thermal shock, dimensional stability andability to transmit radar and infrared fre quencies without appreciabledistortion.

An object of the invention furthermore is to disclose and provide large,dense and uniform articles composed essentially of refractory oxidesfrom the group consisting of alumina, magnesia, titanium and zirconium.

An object of the present invention moreover is to disclose and provide amode of operation and a sequence of steps wherein refractory oxides infinely divided form may be converted into coherent, strong and denseobjects.

A further object is to disclose conditions and steps whereby firingshrinkage of refractory ceramic objects may be greatly reduced.

A still further object of the inventionis to disclose and provide amethod of converting very finely divided refractory oxides into formedobjects, without the necessity of employing plastic clays or low meltingalkalis and reagents, and insuring uniformity by a sequence ofvacuumizing and hydropressing steps.

These and various other objects, advantages and adap tations of theinvention will become apparent from the following description of certainexemplary bodies and modes of procedure. In order to illustrate andfacilitate understanding of the invention, reference will be had to theappended drawings, in which:

FIG. 1 somewhat diagrammatically represents an initial step in theformation of a tubular object in accordance with the method hereindisclosed.

FIG. 2 is an enlarged side elevation partly in section of a tubularobject being subjected to a vacuumizing step after the initial formingoperation.

FIG. 3 diagrammatically illustrates a hydropressing step in the process.

As previously indicated, the most desirable and useful refractory oxidesare those of alumina, magnesia, zirconia and titanium. Alumina, A1 0 ispreferably used in a prefused and then ground condition. It is veryrefractory and has a melting point of over 3600" F. The magnesia oxideis also preferably electrically fused and finely ground and in suchcondition is stable and relatively inactive and has the same meltingpoint as periclase (about 5000" F). Zirconia, ZrO and titanium oxide arealso adaptable for use in the methods of this invention. In

addition, the body compositions may contain other oxides or salts orcompounds of metals resulting in refractory oxides (such as chromicoxide, manganese oxide, cobalt oxide, nickel oxide, zircon, etc.) orperforming the functions of a mineralizer (such as the halides oflithium, magnesium, calcium, barium, etc.) or acting in both capacities(as in the case of aluminum and magnesium chloride and hydroxide). Verysmall and almost insignificant quantities of other ceramic materialssuch as talc and clay may be added to the compositions, but the totalquantity of talc, clay or other semi-refractory but nonmineralizingmaterials should not exceed 4% and is preferably maintained under 2%.

In all instances the refractory oxides which constitute the major andessential portion of the ceramic body (as well as all other components)are ground to extreme fineness of subdivision. The average particle sizeof such oxides should be finer than material passing a 325 mesh sieveand preferably comprise particles having an average dimension on theorder of less than 25 microns. The particle size of these oxides withinthe range of 25 microns or 10 microns and 1 micron in average dimensionmay be classified or graded so as to obtain a dense resultant structure.Submicron particles are not excluded.

Although in normal ceramic practice, ceramic compo nents are generallysuspended or wetted with aqueous so lutions of water, We have found itdesirable to employ organic liquids, and to suspend the refractoryoxides (and other components) in such organic liquids to form arelatively heavy suspension. The solvent employed should be relativelyvolatile and may be a ketone, an ester, ether or an alcohol. Chlorinatedsolvents and petroleum solvents are not favored but are not excluded. Informing this suspension, it is desirable to employ a small quantity of aresin to impart coherence to the article when first formed and thereforethe solvent utilized should be a solvent for the resin employed. Variousresins may be used, among them being the acrylic, alkyd, vinyl,polyvinyl, polystyrene, cumar indene, ethylcellulose or other cellulosicresins such as cellulose acetate, propionate, etc. It is to be notedmoreover that most of the preferred resins are thermoplastics; thesolvents therefor should have a boiling point not over about 200 F.

The materials hereinbefore referred to are first compounded to form asuspension or slip. Such suspension or slip preferably comprises 60% to80% and generally 65% to 75% of the inorganic solids (which may consistof from between 95% and 99% of the refractory oxides and from 1% to 5%of talc, clay or mineralizers) and from about 18% to 40% by weight of asolution of resin and suspending agent in the selected volatile organicsolvent (such solution containing from to of the resin by Weight ofacetone and from 0% to 10% of a suspending agent). As suspending agentsreference is made to substances such as collodion, ethylor hydroxycellulose, or other cellulosic derivative or material which exerts asuspending and defiocculating effect and will be automatically removedin the early stages of subsequent firing without detrimental effect. Inpreparing such sus pensions, it is desirable to make a relativelyconcentrated resin solution in the acetone, or other solvent, separatelythoroughly wet the inorganic solids other than the refractory oxides ina separate portion of the solvent, combine the wetted inorganic solidswith the resin solution then add the refractory oxides in a state ofvery fine subdivision and finally add whatever suspending agent orwetting agent it is desired to use in the slip. In some cases asurfactant may be added to the solvent before incorporating the solids,to facilitate elimination of air absorbed on the surfaces of the solidparticles. The entire mixture is then thoroughly agitated until ahomogeneous uniform suspension is obtained. As previously stated, allmaterials are less than 25 microns in average dimension.

The suspension is then sprayed upon a rigid hard smooth surfaced mandrelof the desired shape and size,

the mandrel establishing the inner dimensions of the final object to beformed. A mandrel adapted to form a cylindrical retort or furnace tubeis illustrated in FIG. 1 at 10 and it will be noted that such mandrel isprovided with a cylindrical body portion, a curved or convexed top 11and an enlarged base 12 including a locking groove 13. The mandrel maybe made of any suitable metal and in many instances it is desirable tochromium plate the mandrel in order to provide an extremely smooth hardsurface. Such mandrel 10 is positioned upon a rotatable table id in asuitable spray booth and the mandrel is rotated at a relatively slowspeed about its longitudinal axis, say at a speed of from 15 to 30r.p.m.

The refractory suspension, made as previously described, is contained ina pressure vessel 16 provided with an agitator 17, a source of airpressure 18 and an outlet line 19 leading to a spray gun 20 which may bealso provided with an auxiliary high pressure air line 21. It isdesirable to maintain uniform air pressures supplied to the lines 18 and21. The spray gun 20 is preferably provided with a fish-tail type ofnozzle arranged to deliver a spray in a vertical plane and thesuspension is sprayed upon the slowly rotating mandrel, the spray nozzlebeing moved along a plane passing through the vertical axis of themandrel in a uniform manner so as to leave a uniform deposit of the bodymaterial contained in the suspension on the surface of the mandrel. Itis desirable to start the spraying operation with a spray gun at somedistance from the mandrel, that is, at a distance of say 2 to 2 /2 feet,and as the deposit increases in thickness the spray gun may be movedcloser to the surface of the mandrel so that at the conclusion of theoperation the spray gun may be only 6 inches from the surface of themandrel. Since a volatile solvent is employed a relatively dry coatingis aplied to the mandrel, a large proportion of the solvent vaporizingduring its passage from the spray gun to the surface of the mandrel. Thesuspension in tank 16 may be maintained at a constant pressure withinthe range 3 lbs. to 15 lbs. p.s.i., while air at a pressure of 30 to 50p.s.i. is supp-lied to the nozzle through line 21, to insure fineatomization and accelerate the vaporization of the solvent. The veryfinely divided particles of the refractory oxides contained in suchsuspension are therefore impacted upon the mandrel and form a dense,compact coating, which is temporarily bound and held together by thecontent of resin in the suspension. It may be observed that the rate atwhich the mandrel rotates, the pressure of auxiliary air and thedistance of the spray nozzle from the mandrel are correlated to obtain adeposit on the mandrel which is insufficiently wet with solvent toexhibit plastic deformation or movement nor so dry as to producelaminations. The build-up of the deposit is continued and its desiredthickness and uniformity established by a fixed template (or doctorblade) diagrammatically illustrated at 15 in the drawing.

After a deposit of suitable thickness has been thus formed on the rigid,impervious mandrel, the entire mandrel is permitted to stand and dry atroom temperature until substantially all of the volatile materialscontained in the deposit have evaporated. Thereafter the depositedarticle (carried on the mandrel) is encapsulated covered with acompliant, impervious envelope. By referring to FIG. 2, it will be seenthat the mandrel 10 (now covered by the compacted refractory article inits dried form indicated at 26) is enclosed, encapsulated, covered, andsealed within the preferably preformed, compliant, resilient andimpervious envelope 30, which can be made of heavy rubber or rubbercomposition or other suitable, flexible, resilient material. Thisenvelope 30 is provided at its upper end with a conduit connection 31adapted to be operably connected to a source of vacuum. The lower end ofthe envelope 30 extends over the groove 13 formed in the base of themandrel and is sealed to the mandrel by extending into such groove andbeing compresesd therein by means of a suitable clamping or sealing ringindicated at 25. After the envelope has been sealed to the mandrel theconduit 31 is connected to a source of vacuum and the deposited materialon the mandrel is subjected to a vacuum of not less than 25 inches ofmercury and preferably on the order of 30 inches of mercury for a timesufiicient to completely withdraw all the vaporizable components thatmay be contained within the body of material 26. During this applicationof vacuum, it appears that a certain compacting of the material takesplace and all air as well as vaporizable components are removed frombetween the interstices of the particulate matter which makes up thedeposited body 26. Ambient atmospheric air pressure assists incompacting this body.

Thereafter the entire assembly illustrated in PEG. 2 is placed within apressure vessel such as the one diagrammatically illustrated in FIG. 3.The conduit 31 may be provided with a valve immediately adjacent theenvelope 30 and such valve closed, the remaining portion of the conduit31 disconnected and the assembly of mandrel, deposited body and envelopeplaced with the pressure vessel 32, with a sub-atmospheric pressureretained within the envelope. Alternatively, the conduit 31 may remainconnected to the envelope and extend through suitable packing glands 33formed in the cover 34 of the pressure vessel so that continuingevacuation of the material within the envelope can take place during thesubsequent hydropressing. Plug 35 is removed and the pressure vesselfilled completely with a suitable fluid such as glycerine, mineral oilor other hydraulic fluid. The plug 35 is then locked into position andadditional quantities of fluid pumped into the chamber 32 by means ofsuitable heavy pumps. Hydraulic pressure on the order of 30,000, 35,000or 40,000 lbs. per square inch is thus generated within the pressurevessel 32 and imposed upon the outer surface of the envelope 30. Thispressure is preferably raised slowly and gradually over a period ofseveral hours. As previously indicated a constant source of vacuum maybe applied to conduit 31 to the deposited body 26 of refractory oxidesbetween the central mandrel and the outer envelope 30 during the entireperiod that the assembly is subjected to the hydrostatic pressure of thefiuid. After the pressure reaches the desired maximum (as indicated bysuitable pressure valve gauge 36), the pump is discontinued and thepressure gradually and slowly relieved by means of a valve 37 connectedto the inlet line 38.

It appears that during this hydrostatic pressing, the individual grainsand particles of the body 26 which has been deposited upon the mandrelare caused to shift with respect to each other and to come into a stableconfiguration, whereby the particulate matter assumes the densestpossible relationship. This orientation is the result of the uniformlyapplied pressure. The hydrostatic pressing reduces the wall thickness ofthe unfired article by between about 40% to 60%. It may be mentioned atthis time that care should be taken not to permit any of the hydraulicfiuid itself to contact or impregnate the refractory body 26; theenvelope 30 should be sufiiciently thick and strong so as to withstandthe pressures and transmit the hydraulic pressure uniformly over theentire surface of the deposited material 26 on the mandrel.

After the pressure has been relieved and liquid drained from thepressure vessel 32, the entire assembly is removed from the pressurevessel and the envelope 30 removed. It will be found that the formedarticle is dense, dry and readily removable from the mandrel itself.This formed but unfired article is now preferably subjected to a slowbaking operation at temperatures sufiiciently high so as to cock out andvolatilize the small amount of resinous material which may be containedin the body. Temperatures of between 300 F. and 600 F. are adequate forthis baking step. Preferably the tempera t3 tures are raised ratherslowly, say at a rate of about 75 F. per hour until the maximum desiredtemperature is reached whereupon such temperature may be maintained fora period of say one hour and then the piece allowed to cool to roomtemperature.

The resulting baked and formed body may be lightly machined at thisstage, prior to being subjected to a bisque firing. This bisque firingwill vary somewhat in accordance with the particular refractory oxidewhich constitutes the essential component of the formed article, butusually such bisque firing is conducted at maximum temperatures ofbetween about 2000 F. and 2500 F. It may be noted that cylindrical andconical objects are preferably fired while in a vertical position. Afterfiring to the temperatures indicated, the article is permitted to cooland the shaped, bisque-fired article subjected to machining to securecloser tolerances and dimensions if desired. It is to be remembered thatin many instances wall thicknesses must be held to within very closelimits and desired flanges or other lips need be formed on the article.After such machining the article is subjected to a high firing;ordinarily this subsequent firing is to a temperature in excess of 3000F. Articles composed essentially of alumina are normally fired to atemperature of between about 3100 F. and 3200" F. and soaked at thistemperature for one to two hours. The entire firing schedule shouldcover approximately 48 to 60 hours and the piece is then allowed to coolto room temperature and may be surface ground if desired. in everyinstance the final firing is to a temperature and for a time snfiicientto mature the body composition and obtain a sintering which developsoptimum physical properties in the fired article: this firingtemperature will vary with the refractory oxides employed, the characterof the mineralizing agents present and the intended use of the tinishedarticle. Neutral firing atmospheres containing some water vapor appeardesirable with substantially pure refractory oxide bodies. It may benoted that the manufacturing procedure herein discolsed reduces thefiring shrinkage to a very small quantity (one-half or less of thatwhich would normally be expected) thereby permitting the manufacture ofarticles which conform very closely to specified sizes and tolerances.

A specific example directed to the production of radomes for guidedmissiles may be cited. These radomes were formed on mandrels having thedesired conical configuration. The inorganic body materials comprised97% by weight of fused and ground alumina, approximately one-half beingof so-called 500 mesh size and the other half of so-called 900 meshsize. In addition, the body contained 2.25% by weight of a finely groundtalc and 0.75% by weight of plastic kaolin. The suspension was formedwith acetone as the solvent and. a cumarone indene resin was dissolvedtherein together with a small percentage of collodion. The finalsuspension, as it was sprayed, contained 67.4% by weight of inorganicsolids, 25.05% acetone, 2.18% of the cumar resin and 5.37% of collodion(as a solution of 4 g. pyroxylin, chiefiy dinitrocellulose, in 100 ml.of alcohol and ether).

The radomes were manufactured by spraying the suspension upon themandrel in the manner described hereinbefore; the sprayed objects werepermitted to dry for a period of 48 hours at room temperature and thensubjected to vacuum of 30 inches for a period of 20 minutes, the vacuumbeing applied after a butyl rubber envelope had been secured to the baseof the mandrel and enveloped the formed article. The envelope was sealedto retain the vacuum within the envelope; the assembly was then placedin the pressure vessel and subjected to a maximum pressure of 35,000lbs. p.s.i. during a period of two hours. Thereafter the radomes werebaked at an initial temperature of 200 F. and a final temperature of 550F. at which temperature the radomes were held for one hour before beingallowed to cool. The baked formed bodies were then bisque fired to amaximum smassa temperature of 2150 F., machined to secure closertolerances and dimensions in accordance with preformed te 1-- plates andthen retired to a maximum temperature of 3100"- F.

The missile radomes made from the composition stated and in the mannerrecited had a modulus of rupture of 32,000 p.s.i. and a modulus ofelasticity of 420x10 They had an absorption of less than 0.2%(substantially zero) and a bulk specific gravity of 3.55. The dielectricconstant of the bodies at 8600 megacycles and 68 F. was 8.57, and theloss tangent was only 0.0023. These alumina radomes have shownremarkably uniform transmission rates (on the order of above 95 to radarfrequencies Within the X band. The coefficient of thermal expansion at900 F. was 441x and 481x10 in./in./ F. at 1400 F. Particular attentionis drawn to the fact that the bulk specific gravity (density) of thecompleted article was 89.5% of the theoretic density of pure aluminumoxide and 92% of the theoretical density of the refractory oxides in thecomposition.

Fired objects having physical and electrical properties of enhancedcharacteristics are obtained by the virtual elimination of the minorproportion of normal ceramic components (such as talc and clay in theabove example) and the use of mineralizing components. When suchmineralizing components are employed (or when a portion of therefractory oxides are derived from normally Water-soluble compounds suchas halides of magnesium, aluminum hydroxide, or the like), the densityof the hydropressed but unfircd article may be lower and the firingshrinkage higher than in cases where the body was spraydeposited from acomposition containing the refractory metal ions in the initial form ofoxides, but after firing it will be found that the mineraliz-ed bodywill produce an article of higher density. A part of this invention isdirected to the formation of the desired refractory oxides in situwithin the formed object during firing; the nascent or freshly formedoxides are more reactive and appear to consolidate more readily.Mineralizing components (such as lithium fluoride, for example) appearto facilitate the consolidation and absorption of crystals of aluminaand expedite the rate at which bubbles and gases move out of the body.More homogeneous, denser and stronger finished objects are attained.

In accordance with the procedure described hereinabove and the teachingsstated, reactor tubes can be made by spray-depositing a body upon animpervious mandrel from a suspension containing about 70% of inorganicsolids (composed of 99.3% alumina, 0.2% lithium chloride and 0.5%magnesia) in 30% acetone solution by weight of total suspensioncontaining resin and a defloculating agent. The mineralizing componentscan be used directly or they may be mixed, precalcined, reground andthen added to the suspension. After drying, vacuumizing, hydro-pressingand firing to a temperature of 3200 F., the fired objects will exhibit adensity of almost 98% of theoretical density of alumina oxide.

Zirconium oxide articles of exceptional properties have been made fromorganic solvent suspensions containing, as the sole inorganicconstituents, mixtures of zirconium oxide in monoclinic and cubiccrystalline form. An illustrative mixture is 3 parts by weight ofzirconium oxide in cubic crystalline form (stabilized with calciumoxide) to 2 parts by Weight of zirconium oxide in monoclinic crystallineform. The minor amount of CaO may be termed a mineralizing andstabilizing agent. Any of the volatile organic solvents previouslymentioned, containing a desired solvent soluble resin, Wetting agent orsurfactant and detloculant, may be used in forming a suspension which isthen sprayed to form the desired object and the object then handled inthe manner described hereinbefore.

In order to obtain fired, complete articles in which the desiredphysical and electrical properties are developed to their optimum, theinorganic solids content of the suspension should consist of the highestpossible proportion or refractory oxides or source materials for suchrefractory oxides, i.e., it is desirable that 96% to 99% of suchinorganic solids content consists of the refractory oxide or oxides. Themethods herein disclosed are not limited in their usefulness to suchoptimum conditions however and can be used to great advantage withsuspensions wherein the inorganic solids of the initial suspensioncontain or even only 80% refractory oxides, the remainder being composedof less refractory ceramic raw or precalcined materials or metallicoxides, powders, etc., depending upon the use to which the completedfired article is designed.

We claim:

1. A method of making hard, dense, uniformly compacted and strong hollowobjects formed essentially of refractory oxides, comprising: forming asuspension of inorganic solids comprising not less than of finelydivided, substantially inert refractory oxides in an organic solventhaving a boiling point of not over 200 F., said solvent containing anadhesive resin in solution; pressure spraying such suspension upon arigid, smooth-surfaced and non-absorptive mandrel to form a deposit ofdesired thickness of said inorganic solids on the surface of themandrel; drying the deposit material on the mandrel; encapsulating saiddeposited material with a compliant impervious envelope; subjecting thedeposited material to the action of vacuum to remove virtually allvolatile material; then hydropressing the dry deposit while on saidmandrel to compact and orient the refractory oxides in such depositedmaterial; removing the article composed of such hydropressed anddeposited material from the mandrel and subjecting the article to bakingand firing steps.

2. In a method of reducing firing shrinkage during the manufacture ofdense, dimensionally accurate refractory objects composed essentially ofrefractory oxides, the steps of: forming a deposit of finely dividedrefractory oxides upon a rigid, non-absorptive mandrel, said finelydivided oxides being bound into a coherent deposit by a small 1 amountof resinous material; encapsulating said deposited material with acompliant impervious envelope; subjecting the deposited material to theaction of a vacuum on the order of 30 inches of mercury to removesubstantially all volatile material; then subjecting the encapsulateddeposited material, while on said rigid mandrel, to a uniformly appliedpressure in excess of about 30,000 psi. to mechanically compact saiddeposit; and subsequently removing the compacted material and subjectingit to firing at a temperature adapted to sinter and mature the material.

3. A method as stated in claim 1 wherein the major proportion of theparticles of the refractory oxides employed in forming the deposit havean average dimension of less than 25 microns and a proportion of saidparticles have an average dimension of 1 micron.

4. A method as stated in claim 2 wherein the deposit formed upon thenon-absorptice mandrel includes mineralizing agents and materialsadapted to form refractory oxides upon firing.

5. In a method of producing dense and strong refractory articles, thesteps of: forming a suspension of finely divided inorganic ceramicmaterials in an organic solvent containing a resin in solution, suchsuspension comprising between 60% and 80% by weight of the inorganicmaterials, between 2% and 8% by weight of a synthetic organic resin insolution, and between 18% and 40% of an organic solvent having a boilingpoint not over 200 F; spraying said suspension upon a rigid,non-absorptive support to deposit the inorganic solids thereon; dryingthe deposited solids; subjecting the deposited solids to the action of avacuum on the order of 30 inches of mercury to remove substantially allvolatile material; covering the deposited solids with a compliantenvelope; and subjecting the deposited solids while on said rigid sup- 0port through said envelope to fluid pressures of at least 30,000 p.s.i.

6. A method of making hard, dense, uniformly compacted, strong andhollow objects formed essentially of refractory oxides, comprising:forming a suspension of inorganic solids comprising not less than 95% offinely divided, substantially inert refractory oxides in an organicsolvent having a boiling point of not over 200 F., said solventcontaining an adhesive resin in solution; pressure spraying suchsuspension upon the outer surface of a rigid, smooth-surfaced andnon-absorptive mandrel to form a deposit of desired thickness of saidinorganic solids on the surface of the mandrel; drying the depositmaterial on the mandrel; encapsulating said deposited material with acompliant impervious envelope; subjecting the deposited material to theaction of a vacuum to remove virtually all volatile material;hydropressing the dry deposit while on said mandrel to compact andorient the refractory oxides in such deposited material; removing thearticle composed of such hydropressed and deposited material from themandrel; firing said article to between about 2000 F. and 2500 F.;machining said article to the desired dimensions; and then subjectingsaid article to a second stage firing at a temperature in excess ofabout 3000 F.

References Cited in the file of this patent UNITED STATES PATENTS1,281,405 Marquess Oct. 15, 1918 1,346,638 Crook et a1 June 13, 19201,795,875 Maynard Mar. 10, 1931 1,862,191 Meth June 7, 1932 2,091,569Ridgway et al Aug. 31, 1937 2,152,738 Jeiferey Apr. 4, 1939 2,270,075Miller I an. 13, 1942 2,272,338 Fessler et a1 Feb. 10, 1942 2,781,273Koch Feb. 12, 1957 2,809,126 Murphy et a1 Oct. 8, 1957 OTHER REFERENCESKingery: Ceramic Fabrication Processes, John Wiley and Sons (1958), pp.58 and 147-171.

Ceramic Fabrication Processes, pp. 71 and 72, W. D. Kingery, editor,published by Technology Press of MIT and John Wiley and Sons, receivedin Scientific Library, Apr. 11, 1958.

1. A METHOD OF MAKING HARD, DENSE, UNIFORMLY COMPACTED AND STRONG HOLLOWOBJECTS FORMED ESSENTIALLY OF REFRACTORY OXIDES, COMPRISING: FORMING ASUSPENSION OF INORGANIC SOLIDS COMPRISING NOT LESS THAN 95% OF FINELYDIVIDED, SUBSTANTIALLY INERT REFRACTORY OXIDES IN AN ORGANIC SOLVENTHAVING A BOILING POINT OF NOT OVER 200*F., SAID SOLVENT CONTAINING ANADHESIVE RESIN IN SOLUTION; PRESSURE SPRAYING SUCH SUSPENSION UPON ARIGID, SMOOTH-SURFACED AND NON-ABSORPTIVE MANDREL TO FORM A DEPOSIT OFDESIRED THICKNESS OF SAID INORGANIC SOLIDS ON THE SURFACE OF THEMANDREL; DRYING THE DEPOSIT MATERIAL ON THE MANDREL; ENCAPSULATING SAIDDEPOSITED MATERIAL WITH A COMPLIANT IMPERVIOUS ENVELOPE; SUBJECTING THEDEPOSITED MATERIAL TO THE ACTION OF VACUUM TO REMOVE VIRTUALLY ALLVOLATILE MATERIAL; THEN HYDROPRESSING THE DRY DEPOSIT WHILE ON SAIDMANDREL TO COMPACT AND ORIENT THE REFRACTORY OXIDES IN SUCH DEPOSITEDMATERIAL; REMOVING THE ARTICLE COMPOSED OF SUCH HYDROPRESSED ANDDEPOSITED MATERIAL FROM THE MANDREL AND SUBJECTING THE ARTICLE TOBRAKING AND FIRING STEPS.